Data communications over communications systems have traditionally been performed using a modem. Data communications over a public switch telephone network (“PSTN”) is performed over a voice channel, as there are no data channels on this communications system. However, cellular networks have both data and voice channels over which data may be communicated.
Data communications, as opposed to voice communications, may be utilized for a variety of purposes. One such purpose is vehicle telematics. Vehicle telematics are generally considered to need immediate communication capability due to various safety and security concerns for drivers. For example, in the case of emergency, a communication may need to be communicated from a vehicle to a call center. Generally, vehicle telematics systems utilize bi-directional data transmission between vehicles and call centers.
Because of vehicle equipment costs constraints, data and voice communications for vehicle telematics generally utilize a cellular communications system. Since “safety and security” is a large part of these systems, data communications between the vehicle and call center should be delivered quickly and reliably. North American vehicle telematics suppliers face the challenge of providing coverage over the entire continent, which necessarily includes coverage using three different existing cellular technologies, which are AMPS, GSM, and CDMA. Two of these cellular technologies, GSM and CDMA, provide data channels for communicating data; AMPS does not provide a data channel as AMPS is an analog communications system. The data channels of CDMA and GSM rely on IP protocols, where message delivery time may range from several seconds to several hours, thereby rendering the data communication path inferior for safety and security purposes of vehicle telematics or other uses of data communications that have a need or desire for substantially real-time communications. Furthermore, different cellular technologies use different protocols and equipment, thereby adding complexity to call centers. As a result, timely data transmission is not possible utilizing data channels of existing cellular technologies and a need exists for data communications that can deliver data quickly and easily over all cellular technologies.
The voice channel of all cellular communications systems provides a low-delay audio path between users, and audio is a common element between all cellular technologies. It is, therefore, natural to use this path for data communication, such as vehicular telematics, to avoid complications using data channels because of timeliness of the data channels and the fact that AMPS does not include a data channel.
While other data communications have attempted to utilize the voice channel for communicating data over a cellular network, these attempts have been or will be problematic due to configuration limitations of the voice channels that are inherent in cellular communications as well as other communications systems. Nearly all cellular telephones in use today use digital voice compression in order to efficiently utilize their allocated radio spectrum. The cellular system component that compresses speech prior to transmission and then expands it after reception is called a voice compressor or a vocoder. As understood in the art, vocoders are located within mobile phones and base stations for transmitting and receiving. Transmitting modulated data through a vocoder using traditional magnitude and phase modulation is impractical because the vocoder preserves neither, as magnitude and phase modulation are not critical for the psycho-acoustical process of human voice communication. Furthermore, traditional magnitude and phase modulation for data communication is frequently recognized by the transmitting vocoder as a non-voice signal and filtered out prior to transmission, thereby making the data-bearing signal unavailable at the receiver.
Conventional modems use various combinations of amplitude and phase modulation to transmit data. Examples are Binary Phase Shift Keying (BPSK), Quadrature Phase Shift Keying. (QPSK), Quadrature Amplitude Modulation (QAM), Minimum Shift Keying (MSK), Gaussian Minimum Shift Keying (GMSK), Frequency Shift Keying (FSK), and Amplitude Shift Keying (ASK). One drawback of these techniques is that they encode information using amplitude and phase. Since the psycho-acoustical process of human hearing is relatively insensitive to both of these parameters, voice compressors (i.e., vocoders) do not preserve amplitude and phase, and therefore, the amplitude and phase information is not transmitted reliably.
Another class of modems uses Orthogonal Frequency Division Multiplexing (OFDM). This modulation transmits multiple carriers simultaneously with each carrier modulated with QAM modulation. This modulation further transmits with all carriers simultaneously on. OFDM is unsuitable for transmitting through vocoders because the QAM modulation on each carrier is not preserved by the vocoder.
Dual Tone Multi Frequency (DTMF) can be used to transmit data over audio channels. This modulation uses two groups of four tones and each symbol consists of one tone from each group, resulting in four information bits per symbol. DTMF tone frequencies were specifically designed to avoid harmonic relationships. One drawback of DTMF is that since the frequencies are not harmonically related it is difficult for the pitch predictor in the vocoder to represent them. A second drawback is that DTMF frequencies occupy a relatively narrow (697-1633 Hz) frequency range and can be attenuated or eliminated by the noise canceller in the vocoder.
One commercially available modem uses the presence or absence of a set of four candidate tones to encode four bits per symbol. This modem is conventional because it simply uses four Amplitude Shift Keying (ASK) carriers. One drawback of this modem is that the amplitudes of the tones are not represented accurately by the vocoder, which makes it difficult for the demodulator to detect whether a tone was sent or not. Individual tones are attenuated by the vocoder based on the composite structure of the signal and history of past signals and future of incoming signals cannot be relied on as the amplitude is inaccurate. The receiver using threshold detection to detect the presence or absence of a tone is similarly inaccurate due to the amplitude being inaccurate.
Current vocoders compress voice at bit rates between 2,000 and 14,000 bits per second (“BPS”). The entropy or information content of toll-grade human speech is much lower (e.g., between 100 and 200 bits per second), suggesting that future vocoders will operate at much lower rates. There are currently several modems available for use with cellular telephony. These modems use conventional modem waveforms that are not producible by the human vocal tract, and they send data at rates well above the entropy rates. This suggests that these conventional modems are taking advantage of the inefficiency of state-of-the-art vocoders, and will unlikely operate correctly when the cellular operators adopt newer, lower rate vocoders. Deployment of these conventional modems has been limited for this reason.